Ceramic insulating material



Patented Apr. 25, 1939 George L. McCreery and Van E. Campbell,

Barberton, and Ray Higgins, Wadsworth, Ohio, assignors to The Ohio Brass Company, Mansfield, Ohio, a corporation oi" New Jersey No Drawing.

' 11 Claims.

This invention relates to ceramic insulating material and particularly insulating material for condensers, capacitance couplers and other electrical apparatus requiring insulation having relatively high specific inductive capacity and resistance to puncture.

One object of the invention is to provide an insulating material which will have a high specific inductive capacity and which may be economically manufactured and formed into desirable shapes.

A further object of the invention is to provide .a material of the class named in which the shrinkage, due to drying and firing, will not be m excessive.

A further object ofs the invention is to provide a dielectric material in which the specific inductive capacity will not increase with rises in temperature.

Other objects and advantages will appear from the following description.

The invention is exemplified by the combination of materials described in the following specification and by the process of combining'and treating the same, and it is more particularly pointed out in the appended claims.

For the manufactureof electrical apparatus such as condensers and insulators used as condensers and capacitance couplers, it is desirable to have an insulating material or dielectric with low power factor, small phase angle and high specific inductive capacity. It is also necessary for high voltage condensers to have a high puncture strength, resistance to weathering and freedom from burning under momentary fiashover.

Electrical porcelain possesses the last three properties to a high degree but has been under some disadvantage because of a relatively low specific inductive capacity or'electrostatic constant commonly designated by the letter K.

The K of a porcelain-like body may be varied by the incorporation of mineral oxides or silicates having in themselves a big. specific inductive capacity. Titanium oxide, of which the crude ore is called rutile, possesses a high K but it is a very refractory mineral andits employment for a high voltage insulator has been quite difficult. The dimculty has been to find a combination of ingredients which would make a workable mass in the unfired state and produce a dense non-porous insulating material when fired.

Compositions have been prepared for manufacture of small parts such as short wave radio condensers in which ground rutile was combined as with steatite or talc as the principal fluxing in- Application December 24, 1936, Serial No. 117,558

gredient. vThe combination is handicapped by a very large shrinkage,amo1inting to 30 percent or more, by the time the piece is fired. To meetwthis dificulty and produce ware of uniform dimensions, ithas been necessary first to fire the pieces 5 at a temperature high enough to vitrify them partly and reduce the shrinkage to a known I amount, then trim the parts to a definite size and re-fire at a temperature high enough to complete the shrinkage and produce a non-"porous 10 body. The trimming operation is not easy because this material in the partly vitrified state is so hard and abrasive that even the hardest alloys used for metal cutting are rapidly worn out. The double firing operation adds greatly I to the cost of manufacture and the firing temperatures necessary to mature the rutile with a talc or steatite flux have been in the ranges of cones 14 to 18 (2500 F. to 2700 F.) which adds greatly to the cost of suitable kilns and the ex- 0 pense for fuel.

We have discovered other body compositions which reduce the shrinkage to approximately 10%, maturing at temperatures in the ranges of cones 10 to 12 (2300 F. to 2400 F.) and by their 25 smaller and more uniform shrinkage eliminate the intermediate trimming and second firing operations unless parts with exceptionally close tolerances in dimensions should be required.

We have discovered that crude rutile when 30 finely ground has a variable but definite flocculating action upon a slip of clays, flint and feldspar as used in preparation of electrical porcelain, such that the slip is rendered highly viscous and. diflicult to work even though the proportion of 35 solids is not more than 50 percent of the total weight. Ware prepared from such mixes by any of the usual wet or plastic processes has very great drying and firing shrinkages. The usual deflocculating agents, suchas sodium silicate 40 which is often added to correct such troubles, caused blistering when fired. However, if the rutile has been previously roasted at a temperature close to the sintering point, this fiocculating property'of the titanium oxide is greatly re- 45 duced. Such a prepared titanium oxide may be obtained under the designation of heavy grade titanium oxide such as is sold by the Titanium Alloys Manufacturing Company, of Niagara Falls, New York. With this roasted titanium oxide, 50 slips may be prepared containing 70percent total solids, having suitable working properties, producing ware with a dry to fired shrinkage in the order of 10 percent.

The electrical properties of these mixtures may be varied to some extent by the proporhon of heavy grade titanium oxide used. Examples 1 and 2 illustrate two typical compositions of this sort and Table 1 shows measurements of their electrical properties as compared with porcelain.

Example 1 Y Per cent by weigh 'IiOz (pre-roasted) 30.0 Ball clay 24.0 China clay 6.8 Flint 5. 2 Feldspar 34.0

100. 0 Example 2 Per cent by weight T102 (pre-roasted) 40.0 Ball clay 30.0 China clay 3.4 Flint 2.6 Feldspar 24.0

100.0 TABLE 1 Electrical properties of 6" a: V discs filifilifi. 35; mg; 5323?? "tilnt am am an: cycles) cycles) cycles) 60 cycles) Volta per Percent mil Example 1 16. 5 046 1. 35 178 Example 2 21.0 039 1. 15 171 Electrical porcelain. e 3 136 2.70 200 It is not intended to limit the scope of the invention to the exact proportions given in Examples 1 and 2 but to point out that by the use of the pre-roasted titanium oxide a series of workable mixtures may be prepared with the clays,

flint and feldspar ordinarily employed for elec-- trical porcelain and still miaintain low shrinkage.

In mixtures of this type the best results are obtained when the amount of pre-heated titanium oxide is within the range of from 10% to 50% by weight of the total solids. However, when it is desired to take more advantage of the specific inductive capacity of the titanium oxide, it is necessary to increase the proportion of this ma.- terial to more than 50% of the mix. It then becomes necessary to introduce an electrolyte or deflocculating agent into the slip which will enable the relatively small proportion of plastic ingredients to hold the titanium oxide in suspension while the mixture is in the liquid state and to retain the shape of the piece while it is being dried, trimmed and in, the early stages of firing. It is also desirable that the fluxing ma.- terials, which must be added should be in such small amounts that their effect will not materially reduce the specific inductive capacity introduced by the titanium oxide.

We have found that an alkali titanium silicate, such as potassium or sodium titanium silicate, when finely ground in water, possesses both the property of a suitable electrolyte or defiocculating agent and an eflicient fluxing material when used in proportions up to 3 percent of the weight of solids. By the term alkali titanium silicate we do not include compounds of titanium and silicon with the alkaline earths, such as calcium,

areaese gnesium or barium, as we have found that they do not produce the results obtained by the alkali compounds. Examples 3 and 4 illustrate typical mixtures and Table II shows measurements of their electrical characteristics compared with electrical porcelain Earample 3 Per cent 1 by weight Titanium oxide (pm-roasted) 68 Ball clay 30 Sodium titanium silicate 2 100 Example 4 Per cent by weight Totanium oxide (pre-roasted) 83 Ball clay 15 Sodium titanium silicate 2 100 TABLE II Electrical properties of 6" a: A" discs Specific ggggggg it??? i123. will;

(at 1000 (at 1000 (at1000 disc) (at cycles) cycles) cycles) 60cycles) Valle per Percent mil Example 3 32. 0 039 1.13 172 Example 4 53. 0 041 1. 19 162 Electrical porcelain. 6. 3 136 2. 70 200 It is not intended to limit the scope of the invention to the proportions shown in Examples 3 and 4 but to illustrate the.possibilities of the combination to produce ceramic insulating ma.- terials with a Wide range of specific inductive capacities by modifying the proportion of preroasted titanium oxide and ball clay. When preroasted titanium oxide is used with sodium titamum silicate as a deflocculating and fiuxing agent, the best results are obtained if the amount of pro-roasted titanium oxide does not exceed by'weight of the total solids.

One great advantage of the mixtures described above is that they may be worked much more readily, and with a. smaller proportion of water in the mix, than is possible with previous mixes containing titanium oxide. In forming pieces by casting, this is of particularly great advantage because of the smaller amount of water in the slip to be absorbed by the mold. In the process of casting ceramic pieces, we have found that the use of an alkali titanium silicate, and particularly sodium titanium silicate as a deflocculating agent, greatly facilitates the casting process and also serves as an efficient fiuxing agent when the piece is fired.

One further advantage of the dielectric material made according to the present invention is that it may be used for controlling the efiect of temperature changes upon the specific inductive capacity of the dielectric. Titanium oxide has a negative coefilcient of specific inductive capacity for temperature changes while that of porcelain is positive, so that by varying the proportions of titanium oxide and clay, it is possible to control the coemcient as desired. Where a dielectric having a large proportion of titanium oxide is used, if it is desired to ofiset the coefllcient, this may be done by using it in conjunction with porcelain or other dielectric material having a positive coemcient.

We claim:

1. Ceramic insulating material of high specific inductive capacity in which pre-roasted titanium oxide is combined with clay using sodium titanium silicate as an electrolyte and fiuxing agent.

2. Ceramic insulating material comprising clay, pre-roasted titanium oxide and sodium titanium silicate.

3. Ceramic insulating material comprising titanium oxide and alkali titanium silicate.

4. Ceramic insulating material comprising titanium oxide and sodium titanium silicate.

5. Ceramic insulating material comprising titanium oxide, sodium titanium silicate andclay, the titanium oxide constituting more than 40 percent of the total mixture and the sodium titanium silicate approximately 2 percent of the total mixture.

6. Ceramic insulating material comprising titanium oxide, sodium titanium silicate and clay, the titanium oxide comprising from 40 to percent of the total. v

7. In the process of casting ceramic bodies, the

step of using alkali titanium silicate as a deflocculating agent in a wet mixture.

8. In the process of casting ceramic bodies, the

step of using sodium titanium silicate as a deflocculating agent in a wet mixture.

9. The method of manufacturing dielectric material of high specific inductive capacity comprising the steps of heating rutile to a temperature slightly below the sintering point to provide pre-roasted titanium oxide, mixing the preroasted titanium oxide in a finely ground state with clay, water and sodium titanium silicate to form a plastic mix, and firing the mix to produce vitrification.

10. The process of preparing ceramic material comprising the step of defiocculating a mixture of ceramic material and water with alkali titanium silicate.

11. The process of preparing ceramic material comprising the step of deflocculating a mixture of ceramic material and water with sodium titanium silicate.

GEORGE L. McCREERY, VAN E. CAMPBELL, RAY HIGGINS. 

